Strong acid cation (SAC) exchange resins commonly are used in water treatment systems and methods. These resins may be present in filters through which treated water has been passed to remove contaminants such as metal cations, which contribute to water “hardness.” As such, SAC exchange resins commonly are used in water “softening” applications where water hardness must be treated to prevent significant end-use problems such as scale and soap-scum formation, excess detergent use, corrosion of household fixtures, and reduced life of household appliances. Commercial applications of water softening have these same concerns and many others as well. In the food service and beverage dispensing industry, water must be treated not only to meet the specified optimum degree of water softening, but also to obtain water that is clear and colorless so as to not alter the quality and appearance of the food or beverages produced from the treated water.
One disadvantage of using SAC exchange resins in water treatment systems is that these resins continuously leach material into the treated or “effluent” water. This leached material or “leachate” may impart undesirable properties to the effluent water, such as undesirable color. Significant concentrations of leachate may be released from the SAC exchange resin during periods of non-use, such as non-use during storage or retail display, overnight non-use, or down time non-use during equipment servicing.
In particular, synthetic SAC exchange resins that include divinylbenzene (DVB) cross-linked styrene sulfonic acid continuously leach an intensely colored species of non-polymerized (linear) styrene sulfonate. This leachate is a leftover by-product of the manufacturing polymerization process, but also may be generated from the resin beads themselves during use, due to the breakdown of the resin in oxidative reactions. Depending upon the concentration of the leachate material that has built up within the filter, this leachate material may stain the filtered water anywhere from a slight yellow color to a dark brown color. This causes a significant negative aesthetic effect when a consumer attempts to use the treated water. This negative anesthetic effect is particularly undesirable for end products in the commercial food and beverage preparation industries.
Some systems have attempted to remove negatively charged (anionic) leachate material from an SAC exchange resin by subsequently treating the effluent with a separate bed of anion exchange resin. However, these systems have inherent limitations because they require the use of an additional resin bed, thus increasing the size of the filter. Furthermore, the anion exchange resin bed may have poor kinetic absorptive capability for leachate material and may only remove the leachate after the leachate contacts the anion exchange resin for a sufficient period of time. The dynamic flow rates used in standard SAC exchange resin filter systems typically exceed the slower flow rates required for a separate anion resin bed to absorb cation resin leachates. Therefore, better systems for removing SAC exchange resin leachates are desirable.